Magnesium oxide tablets are widely used as medical laxatives, mineral supplements, etc. (for example, Patent Literature 1 to 3).
Generally, tablets are manufactured by compression molding of raw materials. This step of compression molding is called tableting step. In a tableting step, compression action is continuously repeated at high speed. Under such conditions, stress and/or density distribution occurs in the tablets, resulting in heterogeneous inner structure. The heterogeneity is considered to be the primary cause of tableting problems and quality defects, etc.
Examples of the tableting problems include capping, laminating, sticking, picking, etc. Capping is a phenomenon in which the top of a tablet separates from the rest and comes off like a cap, which occurs after the molding, during or after the ejection of the tablet from the die. Laminating is a defect even worse than capping and is a phenomenon in which a tablet (not the top but the intermediate part) separates into horizontal layers. Sticking is a phenomenon in which powder adheres to the punch (compression rod) during the compression step and cause cavity-like defects on the surface of a tablet. Chipping is a phenomenon in which the surface of a tablet is slightly damaged for a similar reason. These tableting problems are chiefly attributable to the characteristics of the raw materials.
Examples of the quality defects include tablet defects including cracking, chipping, or dust fall that occurs even in usual usage conditions. Such quality defects mainly result from insufficient strength (hardness) of the tablets.
Also in the production of magnesium oxide tablets, in particular when magnesium oxide of high purity is used as a raw material, there arise the above tableting problems and quality defects.
Magnesium oxide is produced by the calcination of magnesium carbonate or is produced by the addition of calcium hydroxide to sea water or an aqueous solution of magnesium chloride (bittern or salt water) to give magnesium hydroxide, followed by filtration, drying, and calcination (Patent Literature 4).
The above-mentioned magnesium oxide produced by the calcination of magnesium hydroxide has varied characteristics depending on the calcination temperature, and is used for different applications. For example, magnesium oxide produced by calcination of magnesium hydroxide at high temperature of 1500° C. or higher has little activity and has high-temperature resistance and fire resistance. Such magnesium oxide is used as a refractory material, that is, as a raw material for basic refractory bricks, unshaped refractory, etc. Magnesium oxide obtainable by calcination of magnesium hydroxide at 450 to 1300° C. has relatively high activity, and is used as a raw material for magnesia cement, as an additive (mineral source) for fertilizers and foodstuffs, as a raw material for medicinal products, such as an antacid and a laxative, etc. This type of magnesium oxide is, when used as an additive for fertilizers and foodstuffs or as a raw material for medicinal products, directly or indirectly taken into the human body; or when used as a raw material for cosmetics, brought into contact with the human body. Therefore, magnesium oxide having less heavy metal incorporation (less heavy metal content) and having high purity is desired.
As described above, there has been a desire for the development of magnesium oxide that can be preferably used for pharmaceutical applications or as a food additive, has high purity, and can be tableted with reduced problems.